Process for manufacturing poly(lactic acid) bio-composites....

ABSTRACT

Disclosed is a method for manufacturing bio-composites, in which biodegradable poly(lactic acid) (PLA) fibers are mixed, optionally along with general-purpose polypropylene fibers, using a carding process, and compression molded into bio-composites which overcome the problems of the PLA bio-composites manufactured by injection molding, and the PLA bio-composites manufactured thereby.

TECHNICAL FIELD

The present invention relates, in general, to a method for manufacturingbio-composites and, more particularly, to a method in whichbio-degradable poly(lactic acid) (PLA) fibers are mixed, optionallyalong with general-purpose polypropylene fibers, using a cardingprocess, and compression molded into bio-composites which overcome theproblems of the PLA bio-composites manufactured by injection molding.Also, the present invention is concerned with the PLA bio-compositesmanufactured by the method of the present invention.

BACKGROUND ART

With an increasing concern being taken in global environmental problems,such as the treatment of waste plastics, and the sway of the ClimaticChange Convention, new environmental regulations, etc., intensiveattention has been paid to new environmentally friendly materialsincluding bio-composites. Bio-composites are generally composed ofnatural cellulosic flour, such as wood flour, chaff flour, bamboo flour,etc., and reinforcements made of natural fibers such as wood fibers,linen, hemp, etc. As such, bio-composites are used as substituents forconventional polymer composites composed of inorganic materials such ascarbon fiber using glass fibers as reinforcements. Compared to theinorganic fillers, bio-composites have the advantage of beingbio-degradable and friendly to the environment.

DISCLOSURE Technical Problem

Most of the currently used or studied bio-composites are based onpolyolefins (PP, PE, PS), which are most widely used in the polymerindustry, with environmentally friendly, biodegradable natural fibers orflour used as a reinforcing agent. These bio-composites are developedfor use in and can be found in deck construction materials, structuringmaterials, packaging materials and materials used in the interiors ofcars. However, polyolefin-based composites, although partiallyenvironmentally friendly, are not regarded as being environmentallyfriendly due to the non-biodegradability of the polyolefin base. In thefuture, thus, completely biodegradable bio-composites based onbiodegradable polymers are expected to find various applications inindustry. Active studies are now being conducted to bestow onbio-composites physical properties as good as those of the currentlyused general-purpose resins.

Of the many biodegradable polymers, PLA (poly(latic acid)) attractsintensive attention. The use of biodegradable PLA as a base polymer forbio-composites is advantageous in that when buried in a landfill thepolymer is degraded to non-toxic materials. In addition, PLA is asustainable bio resource which can substitute for exhausting petroleumresources. However, the high brittleness of PLA fibers at roomtemperature causes problems when prepared into bio-composites throughinjection molding. That is, when PLA in mixture form with naturalfillers having a higher Young's modulus is injection-molded, theresulting bio-composites are apt to break. In addition to thisbrittleness, the natural material shares the problems attributable tothe flour processing needed by injection molding.

In order to overcome the problems occurring when PLA fiber-basedbio-composites are prepared by injection molding, the present inventorssuggest a compression molding using a carding process. Further, thebio-composites prepared from PLA in combination with the general-purposepolymer PP by compression molding score well on various physical indexesand overcome the brittleness of PLA.

Technical Solution

It is an object of the present invention to provide a method formanufacturing a bio-composite through compression injection using acarding process by which the physical properties overcome thebrittleness of PLA.

It is another object of the present invention to provide a method formanufacturing from PLA fibers a bio-composite improved in physicalproperties by manufacturing it in combination with the general-purposepolymer PP through compression injection using a carding process.

It is a further object of the present invention to providebio-composites which can find application in various industrial fieldsrequiring mechanical strength, including cases for electronic appliancesand the interiors for cars.

ADVANTAGEOUS EFFECTS

The bio-composite manufactured using the method comprising: mixingpoly(lactic acid) (PLA) fibers as a base polymer and natural fibers as areinforcement through a carding process to give webs; processing thewebs under a predetermined pressure into a mat; and compression moldingthe mat to a bio-composite plate which overcomes the problem of highbrittleness of PLA and meets required physical properties includingtensile strength, flexural strength and impact strength.

In addition, the method of the present invention guarantees strength andbiodegradability in the bio-composites even if PLA fibers are used incombination with PP fibers. Having a certain degree of mechanicalphysical properties, the bio-composites of the present invention findapplication in various industrial fields requiring mechanical strength,including cases for electronic appliances and the interiors for cars.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram schematically showing processes ofmanufacturing bio-composites in accordance with the present invention;

FIG. 2 is a schematic view showing a method of measuring the flexuralstrength of bio-composites in accordance with the present invention;

FIG. 3 is a graph showing tensile strengths of bio-composites accordingto types of base polymer and the contents of natural fibers;

FIG. 4 is a graph showing tensile strengths of bio-composites accordingto types of base polymer;

FIG. 5 is a graph showing flexural strengths of bio-composites accordingto types of base polymer;

FIG. 6 is a graph showing impact strengths of bio-composites accordingto types of base polymer and the contents of natural fibers; and

FIG. 7 is a graph showing impact strengths of bio-composites accordingto types of base polymer.

BEST MODE

In order to above objects, there is provided a method for manufacturinga bio-composite, comprising: mixing poly(lactic acid) (PLA) fibers as abase polymer with natural fibers functioning as a reinforcing materialin a carding process to produce webs; processing the webs under apredetermined pressure into a mat; and compression molding the mat to abio-composite plate.

Also, provided is a method for manufacturing a bio composite,comprising: mixing poly(lactic acid) (PLA) fibers as a base polymer withnatural fibers functioning as a reinforcing material in combination withpolypropylene fibers in a carding process to produce webs; processingthe webs under a predetermined pressure into a mat; and compressionmolding the mat to a bio-composite plate.

The bio-composites overcome the problem of high brittleness of PLA andmeet required physical properties including tensile strength, flexuralstrength and impact strength.

In addition, the method of the present invention guarantees strength andbiodegradability in the bio-composites even if PLA fibers are used incombination with PP fibers. Having a certain degree of mechanicalphysical properties, the bio-composites of the present invention findvarious industrial applications including electronic appliances andinterior materials of cars.

A better understanding of the present invention may be obtained throughthe following examples which are set forth to illustrate, but are not tobe construed as limiting the present invention

First, a description is given of the components of the bio-compositeaccording to the present invention, and then of the bio-composites andexperiments therewith. As used herein, the term “carding process”, meansa process by which raw fibers in small aggregates are separated intoindividual yarns with the removal of unwanted materials or unwantedshort lengths, and then they are placed in parallel to each other andprepared into slivers. The carding process is conducted using a cardingmachine.

In the present invention, fibrous poly(lactic acid) (PLA) is used as abio-degradable polymer. It is 30 mm long and ranges in melt index from10 to 30 g/10 min (190° C./2,160 g) with a density of 1.22 g/cm3. PLA isrepresented by the following Chemical Formula 1.

A non-biodegradable material used in combination with PLA ispolypropylene (PP) fibers from Kolon, Korea. The PP fibers have adensity of 0.91 g/cm³ and an MFI of 12 g/10 min (230° C./2,160 g) andare 30 mm long.

In accordance with the present invention, PLA and PP fibers mayoptionally be used together with the natural fibers kenaf and jute(imported by Soo Trading Company, Korea) as a reinforcement for thebio-composites of the present invention. Additionally, natural fibers50˜70 mm in length may be used as a reinforcement.

Example 1 Manufacture of PLA Bio-Composites

PLA bio-composites were manufactured using the followingcompression-molding process. First, the bio-degradable PLA fibers and/ornon-biodegradable PP fibers and the kenaf or jute fibers are mixed usinga carding machine. After being punched with a needle punch, the mixedweb thus obtained is processed at 120° C. under a predetermined pressureinto a mat which was then subjected to compression molding. For thiscompression molding, the temperature was set at 200° C. under a pressureof 70 kgf/cm2. Throughout this process, bio-composite plates wereconstructed. They were stored in polyethylene bags in order to preventmoisture from penetrating thereinto. FIG. 1 shows the manufacturingprocesses of PLA bio-composites comprising a carding process. Thecompositions of the bio-composites manufactured by the processes aresummarized in Table 1. The mixture ratios of the component fibers arerepresented by wt %.

TABLE 1 Compositions of Bio-Composites (wt %) Natural Fiber Nos. PPContents PLA Contents Contents 1 70 Kenaf, Jute, Abaca, 30 each 2 50Kenaf, Jute, Abaca, 50 each 3 30 Kenaf, Jute, Abaca, 70 each 4 90 Jute10 5 70 Jute 30 6 50 Jute 50 7 30 Jute 70 8 35 15 Jute 50 9 25 25 Jute50 10 15 35 Jute 50

Experimental Example 1 Preparation of Specimens

From the dried plate, specimens for use in tensile strength and flexuralstrength tests under 5 MPa were prepared using a punching press. Theywere incubated for 40 hours at a temperature of 23±2° C. and an RH of50±5%.

Experimental Example 2 Measuring Tensile Strength of Bio-Composite

The specimens were measured for tensile strength so as to examine theeffects of the carding process and the polymer materials on thebio-composites. Tensile strength was measured according to ASTM D638-03using a Universal Testing Machine (Zwick Co.) at room temperature with across-head speed set at 5 mm/min. An average value of five measurementswas obtained for the tensile strength.

Experimental Example 3 Measuring Flexural Strength of Bio-Composite

The specimens were measured for flexural strength so as to examine theeffects of the carding process and the polymer materials on thebio-composites. Flexural strength was measured according to ASTM D790-03using a Universal Testing Machine (Zwick Co.) at room temperature with acompression speed set at 5 mm/min. An average value of five measurementswas obtained for the tensile strength. FIG. 2 illustrates a three-pointflexural test.

Experimental Example 4 Measuring Impact Strength of Bio-Composite

Impact strength is the energy per area or length which is required tofracture a specimen subjected to shock loading. According to ASTM D256,impact strength is represented by energy per width of impact side.Measurement is generally conducted in three manners: a notch process inwhich a measurement is achieved in the same direction as a notch, anun-notch process in which a measurement is achieved in the directionopposite to a notch, and a final process wherein an impact is loaded ona vertically-standing specimen. In this example, impact strength wastested using a notch process.

1. Tensile Strength According to Polymer Material

FIG. 3 shows tensile strengths of bio-composites composed of PP plusnatural fibers or PLA plus jute fibers. As seen in this graph, thebio-composite composed of natural fiber and PP decreased in tensilestrength with increasing content of the natural fibers. The decrease wasgradual when the content of the natural fiber was below 30%, but steepwhen more contents were used, indicating that when a lot of naturalfibers were added as a reinforcement, the bond between PP and thenatural fibers was weakened and the tensile strength of thebio-composite was greatly decreased. In addition, higher contents ofnatural fibers increase void volumes within the bio-composite, resultingin a decrease in tensile strength, as elucidated in Table 2 below. Voidvolumes were found to increase with an increasing content of naturalfibers. Void volumes interfere with the bonding of PP to jute fibers andblock the transmission of stress, thus causing a decrease in tensilestrength. The content of voids' interfaces comprised voids at interfacesbetween natural fibers and within natural fibers. When prepared intospecimens with a low content of natural fibers, the bio-composite basedon PLA was apt to crack on the surface of the specimens due to thebrittleness of PLA. As a result, low strengths were obtained in thebio-composite. On the other hand, bio-composites with a natural fibercontent of 50% or 70% were measured to be rather high in strength.Further, their strength was higher than that of the bio-composites basedon PP alone. These results explained one of the reasons why a PLApolymer was selected as a base polymer for bio-composites.

TABLE 2 Void Contents According to Contents of Natural Fibers in Jute/PPComposites Types of Bio- Measured Theoretical Composite Density(g/cm³)Density (g/cm³) Void Contens(%) PP fiber 70 + Jute 0.99 1.03 3.2 ± 0.4fiber 30 PP fiber 70 + Jute 1.03 1.11 7.4 ± 0.3 fiber 30 PP fiber 70 +Jute 1.03 1.18 12.3 ± 2.7  fiber 30

FIG. 4 shows tensile strengths according to base polymers used asbio-composites. PP, a mixture of PP and PLA, and PLA were used asrespective base polymers (Type 1: PP fiber 50%+Jute fiber 50%, Type 2:PP fiber 35%+PLA fiber 15%+Jute fiber 50%, Type 3: PP fiber 25%+PLAfiber 25%+Jute fiber 50%, Type 4: PP fiber 15%+PLA fiber 35%+Jute fiber50%, Type 5: PLA fiber 50%+Jute fiber 50%). The tensile strength wasmeasured to be decreased only in Type 5 which used PLA as a sole basepolymer, but showed similar values over the other bio-composites withoutsignificant reduction, indicating that PLA can partially substitute forPP without a significant decrease in tensile strength. That is,bio-composites based on PLA and PP show tensile strengths similar tothose of PP-based composites, but are more biodegradable, and thus canbe used as more environmentally friendly materials.

Also, as proven by the above results, the PLA bio-compositesmanufactured through compression molding using a carding processovercome the brittleness problem which the bio-composites manufacturedthrough injection molding suffer from. In addition, although a portionof the PP fibers was substituted for by PLA, the bio-composites showedsufficient physical properties.

2. Flexural Strength According to Polymer Material

Flexural strengths of the bio-composites manufactured in Example 1 weremeasured and plotted in FIG. 5. As shown, even the use of Jute fibers ata content of 50% in bio-composites composed of PP and PLA makes nodifference in strength over the bio-composites composed of PP and PLA,suggesting that a combination of PP and PLA as well as PLA only can beuseful as base polymers for bio-composites.

3. Impact Strength According to Polymer Material

With reference to FIG. 6, the impact strength of the bio-compositesincreased as the content of natural fibers increased. The improvement ofimpact strength translates into meaning that the bio-composites overcamethe brittleness of the base polymer. The impact strengths of thebio-composites composed of mixed base polymers are shown in FIG. 7. Asapparent from the data of the graph, the bio-composites composed ofvarious mixtures of PP and PLA plus jute fibers were found to havesufficient impact strength.

1. A method for manufacturing a bio-composite, comprising: mixingpoly(lactic acid) (PLA) fibers as a base polymer with natural fibersfunctioning as a reinforcing material in a carding process to producewebs; processing the webs under a predetermined pressure into a mat; andcompression molding the mat to a bio-composite plate.
 2. The methodaccording to claim 1, wherein PLA fibers are used in combination withpolypropylene fibers to give a web.
 3. The method according to claim 1,wherein the natural fibers are kenaf or jute fibers.
 4. A poly(lacticacid) bio-composite material, manufactured by the method of claim
 1. 5.The method according to claim 2, wherein the natural fibers are kenaf orjute fibers.
 6. A poly(lactic acid) bio-composite material, manufacturedby the method of claim
 2. 7. A poly(lactic acid) bio-composite material,manufactured by the method of claim
 3. 8. A poly(lactic acid)bio-composite material, manufactured by the method of claim 5.